1. fun and good exercise, swimming is a very useful skill.

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()being fun and good exercise,swimming is a very useful skill
答案：It
这里是独立主格结构，相当于一个状语从句，这里就等于Because/Since it is fun and good exercise, swimming is a very useful skil.
意为：游泳很有乐趣，是个不错的运动，所以说游泳是项很有用的技能。